The pulp and paper industry is one of the main industries contributing to the gross domestic products (GDP) of Canada and the USA due to their enormous forest resources. However, the pulp and paper industry is currently struggling financially due to strong competition from countries with low labor costs. One strategy to reduce the production costs, and thus to increase the economic benefits of the pulp and paper industry is to utilize their wasted materials more effectively.
The amount of wastewater generated in the pulp and paper industry was estimated as half of all waste effluents released to surface water in Canada. Recently, the capital cost for a lignocellulosic-based wastewater plant with a hydraulic load of 2.15 MMgal/d was estimated to be $49.4 million and the annual chemical cost for this plant was predicted to be $2.83 million.
In the thermomechanical pulping (TMP) process, wood chips are pretreated with steam, which extracts some organic materials, including lignin from wood, and dissolves it in pressate (i.e. the spent liquor (SL) of this process). This extract is sent to a wastewater treatment plant in order to remove the suspended solids and dissolved organic material prior to its discharge. Lignin of SL can be used in the production of value-added products such as carbon fiber, epoxy resins and adhesives. Alternatively, lignin has a heating value of 27 MJ/kg, which equivalently worth $100-300 per oven dry metric ton. Possessing such a high heating value would make lignin as an alternative fuel.
It has been stated that the main source of chemical oxidation demand (COD) of SL is dissolved lignin and its derivatives. In this regard, the COD reduction of lignocellulosic-based wastewater effluent was the subject of several research projects. It has been claimed that, within two stages of anaerobic reactors, 90% of COD from SL was removed at hydraulic retention time of 21 h. Although biological methods are efficient in removing COD, the treated wastewater has color, as not all lignocelluloses will decompose by biological treatments. To improve the COD removal from TMP wastewater, the co-digestion of lignocelluloses with glucose using thermophilic acidogens was suggested in anaerobic reactors. The main disadvantage of such process is the decomposition and thus wasting of the dissolved lignocelluloses in wastewater. In other words, the biological treatment improved the COD removal from wastewater at the expense of decomposing lignocelluloses. Coagulation with metal salts and polymers (mostly anionic) was proposed to improve the removal of lignocelluloses and COD from SL. In one study, the aerobic fermentation of effluent of alkaline peroxide mechanical pulping (APMP) with Aspergillus niger showed 30% COD reduction via adding 1000 mg/l alum, as a coagulant, and 2 mg/l cationic polyacrylamide (CPAM), as a flocculant. In a similar study, almost 90% of COD was removed by adding 4.5 mg/l aluminum sulfate and 2 mg/l CPAM from the secondary treatment of a wastewater effluent. Although coagulation and flocculation treatments are more effective than biological processes for removing lignocelluloses and COD, their operating cost is significant.
Adsorption was regarded as a fast, selective and economical method for lignin removal from spent liquors. In one study, a two stage adsorption process (using activated carbon with the dosage of 1 g activated carbon per 90 g of SL) reduced the lignin, COD and turbidity of SL of TMP by 60%, 32%, 39%, respectively. Fly ash is produced in solid fuel boilers by burning wood residuals, bark or coal. In prior literature, the utilization of fly ash for adsorption of NOx, SOx and several organic compounds (i.e. phenols) from wastewater effluents and air was discussed. It was stated that up to 90% of lignin was removed from a bleaching effluent of a TMP process by treating with 50 g/l fly ash generated in a steam-producing boiler.
Applicant has performed the first study on the application fly ash to the SL of a TMP process. The main focus of the study is on the changes in the COD, turbidity and lignin removals of pulping spent liquor via treatment with fly ash. In this work, operating conditions (treatment time and dosage of fly ash) for removals of lignin, COD and turbidity was optimized. Subsequently, the impact of a two stage process on treating SL was studied under various conditions. Based on the results, an integrated process was proposed not only for decreasing the load to wastewater system, but also for using treated fly ash as a fuel source in the biomass boiler in an effort to improve the energy balance of the TMP process.